Rowland's Thermonielers with the Paris Standard. 7 



Table III. 



Reduction of Air-T Iter mo meter to Absolute Scale. 

 Air-thermometer. Correction to Air-thermometer. 



o o 







10 -0-0028 



20 -0-0048 



30 -0-0061 



40 -0-0067 



50 -0-0068 



4. Instruments Employed. 



In making these comparisons it would have been theoreti- 

 c-ally better to have used the Baudiu thermometers vertically 

 and the Tonnelot thermometers horizontally, or at least both 

 thermometers vertically. But in order to make it possible to 

 remove the Tonnelot thermometers easily and quickly from 

 the comparison- tank in order to take the zeros, and also in 

 order that the readings of the thermometers might be made 

 with a micrometer telescope, it was decided to make the 

 comparisons with the thermometers in a horizontal position 

 as otherwise there would have been serious mechanical 

 difficulties in the construction of the tank. This made it 

 necessary to apply a pressure-correction to the Baudin ther- 

 mometers to make the readings what they would have been 

 if the thermometers had been vertical ; but this correction 

 could be applied with considerable accuracy by measuring 

 the pressure-coefficients. 



In carrying out these comparisons it was found necessary 

 to construct several pieces of apparatus. As some of these 

 differed a little from any that had been used before, it will be 

 necessary to give a short description of them. 



a. Pressure Apparatus. 



The pressure-coefficients were determined in a manner 

 practically identical with that described by Guillaume in his 

 Thermometrie de Precision, p. 104. Matters were so arranged 

 that the thermometer could be subjected alternately to a 

 pressure of 5 or 6 centim. of mercury, and then to the atmo- 

 spheric pressure of 76 centim. The change in pressure was 

 read by means of a mercury manometer, and the change in 

 the reading of the thermometer was also noted. The method 

 of taking the readings will be described later. 



